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What is a high-entropy alloy?

What is a high-entropy alloy?

The development experience of traditional alloys believes that after more metal elements make up the alloy, many brittle intermetallic compounds with complex structures will be formed, which will deteriorate the properties of the alloy. However, in recent years, researchers have discovered that by mixing 5 or more metal elements in equimolar ratio or nearly equimolar ratio, the alloys obtained by smelting have simplified microstructures and are not inclined to distinguish the main elements. There are intermetallic compounds, with nano-precipitates and amorphous structure and other structural characteristics, with high strength, high hardness, resistance to tempering softening, and wear resistance. This type of alloy was originally defined as multi-principal alloy or high-entropy alloy by Ye Junwei of Tsinghua University in Taiwan and others. There is no existing traditional alloy that can have the above excellent properties at the same time. Therefore, high-entropy alloys have extremely broad application prospects and can be widely used in the production of high-strength, high-temperature and corrosion-resistant tools, molds and machine parts. It is a good opportunity to enter the field of high-function, high-value-added special alloy materials.

The characteristics of high-entropy alloys

Since the design concept of high-entropy alloys is different from traditional alloys, multiple elements with equal atomic ratio or approximately equal atomic ratio are selected as the main element, which determines that high-entropy alloys have different characteristics from traditional alloys.
High-entropy alloys have a single crystal structure. A large number of experiments have confirmed that high-entropy alloys can form a single body-centered cubic or face-centered cubic structure phase or a simple mixed-phase structure of body-centered vertical and side-centered cubic, which shows that the atomic ratio is equal or approximately equal without the main element. The principal elements of the slab will dissolve into a simple structure without forming complex intermetallic compounds.
High-entropy alloys will precipitate nanophase structures and even amorphous structures in the as-cast and fully tempered state. When the high-entropy alloy is melted, the contained element atoms are arranged chaotically. During the cooling and solidification process, the diffusion and redistribution of the numerous atoms of each main element will hinder the nucleation and growth of the crystal, which is beneficial to the formation of nanophase. If the cooling rate is fast enough, high-entropy alloys can show a tendency to become more amorphous.
Relative thermodynamic stability. From the thermodynamic relationship-it can be intuitively concluded that for a multi-principal high-entropy alloy, because the number of components is larger, the mixing entropy is also larger, so the Gibbs free energy is lower and the system is more stable.
This characteristic is an intuitive understanding obtained without considering the magnitude of enthalpy and the competitive relationship between and.
The solid solution strengthening mechanism is remarkable. When the crystal structure of the multi-principal element high-entropy alloy is solid solution, due to the large number of elements, the atomic radius of each principal element element is also different, and the lattice lattice occupied is random, so the alloy has obvious solid solution strengthening effect. It is difficult for dislocation movement and crystal plane slip to make the alloy have the characteristics of high strength and high hardness.
High-entropy alloys have higher thermal stability and resistance to high-temperature oxidation. The high-entropy alloy is named for the large degree of disorder of atoms, and there will be greater degree of atomic disorder at high temperatures. Therefore, the high-entropy alloy will become more stable whether it is crystalline or amorphous, and there is still a solid solution strengthening effect. , Can obtain extremely high high temperature strength. Studies have shown that the high-entropy alloy is annealed at 1100°C for 12 hours and then cooled in the furnace without tempering softening (the alloy steel currently used in industry has tempering softening when it exceeds 550°C).
High-entropy alloys have high corrosion resistance. Certain elements in multi-principal high-entropy alloys are easy to form dense oxide films and high-entropy alloys have the characteristics of amorphous, microcrystalline, single-phase, and low free enthalpy, which all contribute to excellent corrosion resistance.

Application areas of high-entropy alloys

Tools for high-speed cutting

High-entropy alloys have high hardness and wear resistance. The as-cast structure hardness of most high-entropy alloys is 600-900HV, which is equivalent to or greater than the hardness of carbon steel and alloy carbon steel after complete quench hardening; changing the content of alloying elements can further increase the hardness of the alloy. In addition, high-entropy alloys usually exhibit high heat resistance. For example, Al 0.3CoCrFeNiC 0.1 high-entropy alloys have been aged at 700 to 1000°C for 72 hours, and the hardness of the alloys has not decreased, but has increased to varying degrees. The effective cutting temperature of ordinary high-speed steels, such as W 18﹑Cr 4﹑V and W 6﹑Mo 5﹑Cr 4﹑V 2, is within 600 ℃. No matter how high the temperature is, the tool will be obviously passivated. In addition, high-speed steel cutting tools sacrifice the plasticity and toughness of steel while obtaining high hardness and high wear resistance. If the steel has poor plasticity and toughness, the tool often breaks, chipping and other failure modes. High-entropy alloys have good plasticity and toughness while obtaining high hardness. For example, after FeCoNiCrCuAl 0.5 is cold pressed at a reduction rate of 50% (that is, the plastic deformation of the cold-pressed alloy reaches 50%), instead of any cracks, there are nanostructures in the dendrite, the size is about several nanometers to several nanometers. At ten nanometers, the hardness of the alloy is further improved. AlCoCrFeNiTi 1.5 is cold pressed at a reduction rate of less than 32%, and it also shows very good ductility. Such a large reduction ratio is unimaginable for high-speed steel. Therefore, high-entropy alloys have obvious advantages when used in the manufacture of high-speed cutting tools. In addition, the success of the magnetron sputtering method to prepare high-entropy alloy coatings can be used to coat a high-entropy alloy film on the surface of ordinary steel tools, and the coating thickness is within 2.5μm. In this way, both good cutting performance can be obtained and costs can be saved.

Various types of tools and molds

The high-entropy alloy has high hardness, high wear resistance, high strength, excellent high temperature resistance and corrosion resistance, making it very suitable for preparing various tools and molds, especially extrusion molds and plastic molds. For example, the compressive strength of AlCoCrFeNiTi 1.5 is as high as 2.22 GPa, and high-entropy alloys containing Cr or Al have excellent oxidation resistance up to 1100°C. Ordinary die steel cannot take into account wear resistance, corrosion resistance, high temperature resistance and good plasticity.

Golf head

High hardness, high wear resistance and low modulus of elasticity make high entropy alloys very suitable for making golf head striking surfaces. The golf head made of high-entropy alloy can hit the ball farther while ensuring a longer service life on the striking surface of the ball head, thereby improving the product grade and increasing the added value of the product.

Fire-resistant skeleton of super high building

In the “9.11” incident in the United States, the overall collapse of the twin towers was largely due to the sharp decline in the strength of the building’s skeleton steel bars after heating, which made it impossible to bear the weight of the building. With the shortage of land resources, there will be more and more cases of building super-high buildings at home and abroad. Therefore, the fire safety of super-high buildings is attracting more and more attention. The high-entropy alloy has extremely high compressive strength and excellent high temperature resistance. It is used as the fire-resistant framework of ultra-high buildings. It can maintain the original load-bearing capacity of the building in the event of an accidental fire and high temperature of the building. Safe, reduce the loss of personnel and property.

Corrosion-resistant high-strength materials for chemical engineering and ships

High-entropy alloys have excellent corrosion resistance. At room temperature, the corrosion resistance of high-entropy alloy Cu 0.5 NiAlCoCrFeSi in 1mol/L NaCl and 0.5mol/L H 2 SO 4 solution is better than 304 stainless steel (equivalent to OCr 18 Ni 9 in my country’s steel grade) ; CuAlNiCrTiSi alloy is more corrosion resistant than 304 stainless steel in 5% HCl solution, and is far more corrosion resistant than A309 aluminum alloy in 10% NaOH solution. Therefore, high-entropy alloys can be widely used for high-strength corrosion-resistant parts on high-pressure, corrosion-resistant chemical containers and ships.

Turbine blades

The good plasticity of high-entropy alloy makes it easy to make turbine blades, and its excellent corrosion resistance, wear resistance, high work hardening rate and high temperature resistance can ensure the long-term and stable operation of turbine blades and improve service safety. Reduce blade wear and corrosion failure.

Electronic devices, communication fields

High-entropy alloys have soft magnetism and high resistivity, so they have great application potential in high-frequency communication devices. It can be used to make high-frequency transformers, magnetic cores of motors, magnetic shields, magnetic heads, magnetic disks, magneto-optical disks, high-frequency soft magnetic films and speakers.

Other

High-entropy alloys combine many excellent properties and can be applied in a wide range of industrial fields. In addition to the above-mentioned fields, high-entropy alloys can also be used as welding materials, heat exchangers and high-temperature furnace materials. High-entropy alloys have a strong ability to form amorphous, and some high-entropy alloys can form amorphous phases in the as-cast structure. In order to obtain an amorphous structure in traditional alloys, a great cooling rate is required to keep the structure with irregular distribution of liquid atoms to room temperature. The research on amorphous metals has only emerged in recent years. Because there are no dislocations in the structure, it has high strength, hardness, plasticity, toughness, corrosion resistance and special magnetic properties, and it is also widely used. The preparation of amorphous high-entropy alloys will undoubtedly further expand the application fields of high-entropy alloys.

Research status of high-entropy alloys

At present, domestic and foreign scholars’ research on high-entropy alloys mainly focuses on the study of preparation methods, and studies the effect of element content on alloy structure and properties for specific alloy systems. The research objects are mainly 5 to 8 element alloys selected from Al, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn; the properties studied are mainly conventional mechanical properties, such as hardness, resistance There are relatively few studies on compressive strength, wear resistance, corrosion resistance, etc., and there are not many data. The research on the micro-mechanism has not really started yet, only scholars from Tsinghua University and University of Science and Technology Beijing have done a few explorations. Judging from the research results, Tsinghua University in Taiwan is still in the leading position in research and has many invention patents. The application potential of high-entropy alloys is huge and the application fields are wide. However, the relevant data of high-entropy alloys are basically still in the laboratory stage, have not yet entered the application field, and have not yet achieved industrialization. On the one hand, because the related research in this field has just started, many people still don’t know about this kind of alloy; on the other hand, there are very few data on this kind of alloy and the data repeatability is not high enough. Take the five-element equimolar ratio high-entropy alloy AlCrCuFeNi as an example. Vacuum arc furnace smelting is also used. The AlCrCuFeNi alloy structure is divided into Figure 1 and Figure 2. A vacuum arc furnace produced by a company in Shenyang is repeatedly smelted 4 times to obtain AlCrCuFeNi button test Similarly, its microstructure is shown in Figure 3. It is not difficult to see that there are certain differences in the AlCrCuFeNi alloy structure prepared by different researchers using the same method, which fails to achieve high reproducibility. The basic premise for a material to be industrialized is that a material with stable structure and performance can be produced through a certain preparation method.

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Figure.1

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Figure.2

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Figure.3

At present, the high-entropy alloys reported in the literature for experimental research, no matter which method is used to prepare, the samples are very small, generally only tens of grams. Therefore, excluding instrument errors, small errors in weighing raw materials when preparing alloys, a small amount of raw materials loss during the preparation process, and different purity of raw materials used may cause the actual alloy composition to deviate far from the design composition. The structure and performance characteristics of high-entropy alloys are determined by their high mixing entropy. The mixing entropy of equimolar ratio alloys is higher than that of non-equimolar ratio alloys, and the structure and properties of alloys may be more sensitive to certain elements. Therefore, in order to make the experimental data of high-entropy alloys truly apply to the actual production field, it is necessary to improve the accuracy of sample matching and improve the repeatability of data. For high-entropy alloys with excellent performance and good economics, the experimental data can be melted into large samples, and then the corresponding performance can be tested to obtain reference data that can be used in actual production.
There are only a few patents related to high-entropy alloys that have been approved in China. They are related to preparation methods. The high-entropy alloys obtained also have the problem of small samples.

Conclusion

High-entropy alloy is a brand-new alloy field. It jumps out of the traditional alloy design framework. It is a special alloy series with many excellent properties. Adjusting its composition can further optimize the performance, so it has a very broad application prospect. Domestic research on high-entropy alloys has just started. Although many researchers have begun to pay attention to the research on such alloys, the relevant data is still in the laboratory stage and has not really entered the practical application stage. If a specific high-entropy alloy can obtain stable, reliable and industrial reference value of experimental data, it will really and quickly promote the research and application of high-entropy alloys, and high-entropy alloys will be seen in various fields of industrial applications .

Source: China Alloy Flanges Manufacturer – Yaang Pipe Industry (www.epowermetals.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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what is a high entropy alloy - What is a high-entropy alloy?
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What is a high-entropy alloy?
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High-entropy alloy is a brand-new alloy field. It jumps out of the traditional alloy design framework. It is a special alloy series with many excellent properties. Adjusting its composition can further optimize the performance, so it has a very broad application prospect.
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